The mainstream of the cleaning liquid used in industrial cleaning machines is hydrocarbon cleaning liquids. Hydrocarbon cleaning liquids have the advantage that they contain neither ozone-depleting substances nor chlorine, and therefore, are not very harmful to the environment or human bodies. Another advantage of the hydrocarbon cleaning liquids exists in that they can be recycled by distillation using the difference in the boiling point from oil components, and furthermore, the clean vapor generated in the recycling process can be utilized for the cleaning and drying of the workpiece (this process is hereinafter called the “vapor-cleaning and drying process”). In the vapor-cleaning and drying process, a workpiece is placed in a cleaning tank for the vapor-cleaning and drying process. After the vapor is introduced into this tank to clean the surface of the workpiece, the tank is rapidly depressurized to suddenly lower the boiling point of the cleaning agent. This causes the bumping and vaporization of the cleaning agent adhered to the surface of the workpiece, whereby the workpiece is dried. In practically used industrial cleaning machines, the vapor-cleaning and drying process for a workpiece is performed as the finishing process after the workpiece has been cleaned with a liquid by being placed in a liquid-cleaning tank holding a hydrocarbon cleaning liquid. A rinsing process using a hydrocarbon cleaning liquid may additionally be performed between the liquid-cleaning process and the vapor-cleaning and drying process.
The cleaning power of a hydrocarbon cleaning liquid depends on the oil concentration in the liquid or its vapor. Furthermore, an increase in the oil concentration of the vapor in the drying process may cause smears to remain. With the repetition of the cleaning of workpieces by the liquid-cleaning process, the oil which is removed as a result of the distillation recycling of the hydrocarbon cleaning liquid used in the liquid-cleaning process accumulates in the distilling tank. This increases the oil concentration in the recycled hydrocarbon cleaning liquid or its vapor. Therefore, it is necessary to regularly boil down the residual liquid in the distilling tank and drain the oil. To “boil down” means to heat the residual liquid in the distilling tank, without the supply of the hydrocarbon cleaning liquid to the distilling tank and that of the vapor from the distilling tank to the cleaning tank for the vapor-cleaning and drying process, so as to vaporize the hydrocarbon cleaning liquid in the residual liquid and condense the residual oil.
The timing of such a boil-down and oil-drainage operation has conventionally been specified for each individual industrial cleaning machine by the manufacturer of the industrial cleaning machine according to the kind of oil to be removed by the machine, use frequency of the oil and other factors. However, in some situations, e.g. immediately after the delivery of the machine, or when the user has changed the kind of oil used for the machining of the workpiece (or other processes), the cleaning or drying performance may become insufficient. In such cases, the oil concentration in the liquid-cleaning tank is measured, and the timing of the boil-down and oil-drainage operation is changed based on the measured result.
Patent Literature 1 discloses a system for determining the concentration of a contaminant dissolved in a cleaning liquid used for the cleaning of a workpiece by measuring an absorbance of the cleaning liquid using ultraviolet radiation having a specific wavelength within a range of 200 to 380 nm, based on a previously created calibration curve which shows the absorbance with respect to the concentration of a known contaminant. This system can be applied to contaminants composed of oil substances, such as cutting oil, working oil, press oil, machine oil, heat treatment oil, grease, and wax. The measurable range of the oil concentration by this system is from 50 to 1000 ppm (1 ppm=1 mg/L). When the concentration exceeds 1000 ppm, the liquid is diluted with a virgin cleaning liquid to a concentration of 1000 ppm or lower levels before the measurement.
Patent Literature 1 also discloses a cleaning system provided with a circulation passage for a rinsing liquid which extends from a second storage tank holding the rinsing liquid and returns to the same second storage tank after passing through a glass cell in an ultraviolet absorptiometer, in order to automatically measure the concentration of the oil remaining in the rinsing liquid used for rinsing a workpiece which has been cleaned with a cleaning liquid. In this cleaning system, the absorbance of the rinsing liquid, i.e. the oil concentration of the rinsing liquid, is continuously measured.